In order to increase the transmission capacity in optical transmission systems the polarisation-multiplex method is used, whereby two signals are polarised orthogonally to one another by an advantageous means and transmitted on the same wavelength.
If polarisation mode dispersion (PMD) occurs, this leads to coherent crosstalk between the signals. Even at low PMD values this crosstalk makes error-free transmission of polarisation-multiplex signals impossible, whereas in transmission systems without polarisation-multiplex such PMD values are still tolerable. Interference makes itself felt both in amplitude modulation (including multistage) and in angle modulation.
European patent application EP 1 202 485 AI discloses a method for transmitting polarisation-multiplex signals in which a signal is divided into two part-signals which are then reassembled into a time-multiplex signal with mutually perpendicular polarisation levels. The time-multiplex method avoids mutual signal interference and halves the data transfer rate in each signal. However the desired doubling of the transmission capacity is not achieved.